Formation of the neural tube, the rudiment of the adult central nervous system, will be examined using the chick embryo as a model system to determine how this fascinating and important process occurs. The proposed project is designed to establish (1) whether cell elongation during formation of the neuroepithelium can occur either in the absence of microtubules or in the presence of intact microtubules treated with very low concentrations of colchicine, and (2) whether bending of the neural plate is accompanied by apical narrowing of neuroepithelial cells. Furthermore, it will be ascertained whether agents which stimulate the functioning of microfilaments, also result in the constriction of the apices of the cells of the neural plate, and whether this process of constriction is inhibited by agents affecting microfilaments adversely. Cytochalasin will be used also to determine which of the three major stages of neural groove closure (i.e., elevation of neural folds, medial covergence of folds, and fusion of folds) is (are) blocked when microfilaments are inactivated, resulting in dysraphic defects of the central nervous system. Finally, the possible roles of changes in the lengths of the phases of the cell cycle in modifying neuroepithelial cell shape during bending of the neural plate will be explored. By these means our understanding of the mechanisms involved in normal formation of the neural tube will be increased and our comprehension of the events which may go awry leading to certain serious, relatively common congenital anomalies should be enhanced.